1. Field of the Invention
This invention relates to a pneumatic tire, and more particularly to a pneumatic tire in which an organic or inorganic fiber cord usable for reducing a tire weight (weight reduction) is used as a cord constituting a bead core instead of steel cord. Also a critical inner pressure for tire burst apt to be lacking in case of using such a fiber cord is increased to a satisfactory level by appropriating a twisting method of the fiber cord.
2. Description of Related Art
Recently, it is increasingly demanded to attain low fuel consumption and the like in vehicles and hence it is strongly desired to reduce tire weight in accordance with the low fuel consumption of the vehicle.
In general, a bead portion of the tire is required to have a high rigidity because it plays a role of positioning and strongly holding the tire to a rim. For this end, a bead core used in the bead portion is usually made from a high-strength steel cord. From a viewpoint of the reduction of tire weight, there have recently developed so-called weight-reduced tires by using an organic or inorganic fiber cord, which is very light and relatively high in the tenacity, instead of the steel cord forming the bead core.
Up to the present, the tire provided with the bead core made from the above fiber cord cannot be put into practice unless a value of total bead tenacity represented by product of the cord winding number for the formation of the bead core and cord tenacity should be made large as compared with the tire provided with the bead core made from steel cord. This is done in order to render a critical inner pressure for burst as an indication for the judgment of safeness into the same level as the tire provided with the bead core made from steel cord.
It is considered that the difference of tensile properties between the steel cord and the fiber cord, particularly the difference of cord properties over a given period until the occurrence of cord breaking-up largely depends upon the critical inner pressure for burst.
In general, the bead core is formed by helically winding and spirally laminating the cord to form a cord arrangement of plural rows.times.columns. In order to simulate the critical inner pressure for burst in the bead core having such a cord arrangement, when stress distribution in a section of the bead core is investigated by raising an inner pressure of the tire, there is a tendency that stress usually concentrates in the cords located at an innermost row of the bead core in a radial direction of the tire. Hence, the stress distribution in the section of the bead core becomes ununiform.
In case of the bead core made from the steel cord, even if a large stress is applied to steel cords located at the innermost row of the bead core in the radial direction of the tire other than steel cords of the other rows as mentioned above, these steel cords of the innermost row plastically deform while creating a constriction or the like over a constant period until the occurrence of cord breaking-up and are further elongated while causing the plastic deformation without immediately causing the cord breaking-up even at the arrival to their cord tenacity. Thus it is possible to impose stress on steel cords located at the remaining rows of the bead core. As a result, the non-uniform distribution of stress in the section of the bead core is corrected to promote the uniformization of stress and hence stress tends to rise at the breakage of the bead core. Therefore, it is considered that when the bead core is made from the steel cord, the critical inner pressure for burst can be maintained at a high level even if the value of the total bead tenacity is decreased to a certain small level.
On the other hand, when the conventional organic fiber cords not attaining to the appropriation of the cord structure are applied to the bead core, such cords merely indicate even elongation against stress until the occurrence of the cord breaking-up but do not have tensile properties as in the aforementioned steel cord. Therefore, when a large stress is applied to the fiber cords located at the innermost row of the bead core in the radial direction of the tire other than fiber cords of the other rows, it is impossible to sufficiently impose stress to the fiber cords of the other remaining rows over a constant period until the occurrence of cord breaking-up as in the steel cord and hence the non-uniform distribution of stress in the section of the bead core is not corrected and there is not caused the same action of enhancing stress in the breakage of the bead core as in the steel cord. For this end, it is considered that the critical inner pressure for burst can not be maintained at a higher level unless the total bead tenacity in the bead core made from the fiber cords is increased to a certain high level.
In the conventional tire using the bead cord made from the fiber cord, the value of the total bead tenacity should be made large only by increasing the winding number of the fiber cord in order to render the critical inner pressure for burst into the same level as in the tire using the bead core made from the steel cord. However, the increase of the winding number of the fiber cord undesirably brings about the increase of tire weight and the rise of the production cost.